Broadly, the present invention relates to the separation of powder particles from an air stream in which the particles are entrained. More particularly, the present invention relates to thermography and especially to the elimination or reduction of static electricity in a centrifugal separator for thermography powder.
Thermography, or raised printing, is carried out by applying a thermography powder, such as a resin-based powder, onto a printed sheet while the ink thereon is still tacky. Sufficient heat is applied to melt the powder, whereupon the powder expands and thus imparts a "raised" effect to the printed areas.
One type of thermography apparatus which has heretofore been employed for carrying-out raised printing operations has included a powder-applying section through which the printed paper sheets are fed. In this section, resinous powder, e.g., a nylon resin, is applied from a hopper onto the printed surface of each sheet and adheres to the still-tacky ink. Thereafter, excess powder is sucked from the sheet by a vacuum pick-up head, leaving powder only on the ink. The sheets are then conveyed through a heat tunnel in which the sheets and powder are heated to the melting temperature of the powder. As the powder melts, it expands and thus "raises" the printing. Thereafter, the sheets are conveyed through a cooling tunnel wherein the raised print is solidified.
In the above-described powder-applying section, the excess powder is sucked from the sheets by means of negative pressure generated by a powder/air separator which then separates the powder particles from the air stream in which they are entrained. The separator operates under conventional centrifugal/cyclone principles wherein the powder/air stream enters a separation chamber in a tangential direction such that centrifugal force impels the powder particles outwardly toward a curved wall of the chamber as the particles travel along the wall toward an exit leading to a discharge valve. The air stream swirls through a center opening in the chamber in cyclone fashion by means of a negative pressure produced by a suction fan. The discharge valve operates to intermittently disperse powder back to the powder distributing hopper.
On occasion, the proper discharge of powder from the separator is hampered by powder particles charged with static electricity. In that regard, the powder particles may pick-up a static charge during travel from the pick-up head and/or through the separation chamber, which results in the particles becoming suspended above the discharge valve. That is, the charged particles are compelled to swirl-about above the valve, rather than gravitating thereto. Hence, the hopper is not replenished at a sufficiently rapid rate.
It is known that particles which possess a static charge can be neutralized by being contacted with gaseous ozone. Ozone is easily formed since it is the by-product of an electrical discharge or spark occurring in air. Ozonators operating under that principal are available commercially. One such ozonator comprises a series of metal pins which project radially from a metal tube. The tube is electrically charged so that the electrical discharges from the pins produce ozone. The placement of such an ozonator within the separation chamber of the above-described powder/air separator of a thermographic printer was tested. However, this failed to produce satisfactory results, mainly because the powder particles were not neutralized at a sufficiently rapid rate. That is, the quantity of powder particles neutralized per unit time was too low.
It is, therefore, an object of the present invention to minimize or obviate problems of the sort discussed above.
Another object is to provide methods and apparatus for neutralizing charged powder particles in a powder/air separator, especially of the type for feeding thermography powder.
An additional object is to neutralize the static charge of powder particles in a cyclone separator in a manner which eliminates a tendency for the particles to swirl-about above the powder discharge.
A further object is to assure that a large quantity of statically charged powder particles are contacted by ozone rapidly enough to prevent any appreciable decline in the powder discharge rate of a powder/air separator.